1. Field of the Invention
The present invention relates to infinitely variable ratio transmission apparatus of the toroidal race rolling traction type, hereinafter referred to as a variator.
2. Background Art
The basic form of variator comprises a toroidally-recessed input disc connected to an input drive shaft and a toroidally-recessed output disc arranged coaxially with respect to the input disc. A plurality of rollers (usually three rollers) is provided in the toroidal cavity defined between the input and output discs and power is transmitted from the input disc to the output disc by means of the rollers. The rollers are mounted in roller carriages which are subjected to transverse forces (usually by means of double-acting hydraulic pistons). The same hydraulic pressure is normally applied to a so-called end load chamber to apply an axial force to one of the discs in order to ensure that the rollers are frictionally engaged with the input and output discs.
Such transmissions are mainly designed for use in relatively high power, high torque applications such as in motor vehicle transmissions. Indeed, in order to be able to handle the levels of power and torque and to provide a more balanced transmission, it is normally necessary to utilise a pair of input discs and a pair of coaxially mounted output discs, defining two toroidal cavities, each of which encloses three rollers. One advantage of using three rollers in each toroidal cavity is that the arrangement is inherently stable in that roller contact at three equally-spaced locations around the discs minimises bending of the variator components and therefore minimises wear. However, it is also normally necessary for each roller to be provided with its own double-acting control piston and for the hydraulic pressures to be controlled by computer.
Whilst the cost of such sophistication is acceptable in motor vehicle transmissions, it discourages the use of variators in less demanding environments.